This invention relates to ammonium aluminum carbonate hydroxide (hereinafter referred to as NH.sub.4 AlCO.sub.3 (OH).sub.2), to a method for the manufacture thereof and to a method for the manufacture of sintered .alpha.-alumina from said NH.sub.4 AlCO.sub.3 (OH).sub.2.
Concerning aluminum compounds of this kind, there have heretofore been known to the art naturally produced dawsonite (a mineral having the composition NaAlCO.sub.3 (OH).sub.2) and synthetically produced NaAlCO.sub.3 (OH).sub.2 and KAlCO.sub.3 (OH).sub.2. A few reports have been published covering conditions for the formation of these aluminum products and the properties thereof.
These substances are synthesized by blowing carbon dioxide gas into a solution of sodium aluminate and a solution of potassium aluminate. In the solution of alkali aluminate which has had the alkali moiety thereof neutralized and has consequently become supersaturated with aluminate ions, NaAlCO.sub.3 (OH).sub.2 or KAlCO.sub.3 (OH).sub.2 is produced when a large amount of HCO.sub.3 .sup.- is brought into existence therein. For this condition to be satisfied, it is necessary to increase the molar ratio of Na.sub.2 O/Al.sub.2 O.sub.3 or K.sub.2 O/Al.sub.2 O.sub.3 in said alkali aluminate solution and blow carbon dioxide gas rapidly into the solution. The properties of the product are variable with the conditions under which the production is performed. The production on a commercial scale, therefore, requires fairly complex control of operational conditions.
The NaAlCO.sub.3 (OH).sub.2 and KAlCO.sub.3 (OH).sub.2 thus produced enjoy excellent filterability and combine docolorizing, deodorizing, deacidifying and heat-insulating functions and, therefore, and finding increasingly more industrial applications.
Moreover, these aluminum compounds exhibit unique properties in their powdery form and are used as the raw materials for the production of .alpha.-alumina, a substance for which there have been found numerous uses. Said aluminum compounds, however, comprise a large amount of involatile alkali components such as Na or K so that in the production of .alpha.-alumina, for example, a preparatory treatment for the removal of such alkali components is necessary. This required treatment limits their uses.
Sintered alumina possesses excellent physical, chemical and thermal properties and, for this reason, is used extensively in various industrial fields. It has heretofore been customary to obtain a compact product of sintered alumina by roasting at elevated temperatures the aluminum hydroxide produced by the Bayer process and thereby converting it into .alpha.-alumina, pulverizing the .alpha.-alumina for a long time, thereafter adding to the pulverized .alpha.-alumina an agent for inhibiting growth of crystals and an agent for accelerating sintering or molding such as, for example, magnesium oxide or clay and finally firing the resultant mixture at high temperatures in the neighborhood of 1900.degree. C. where there is desired a high-purity sintered alumina having not less thand 99% of alumina content or at temperatures around 1550.degree. C. where there is desired a low-purity sintered alumina having about 85% of alumina content.
This method, however, has the following disadvantages: Since the .alpha.-alumina produced by the Bayer process consists of coarse grains, it entails the disadvantage of necessitating a long time of pulverization. At the same time, during the prolonged pulverization, the substance rubbed off of the wall of the pulverizing machine mingles into the particles under treatment and consequently degrades the purity of the pulverized alumina. Even if the pulverization is carried out amply, it is difficult both technically and economically to effect to size reduction beyond a certain level as, for example, to the extend of increasing the proportion of particles measuring not more than 1 .mu.m to at least 50%. Thus, the pulverized particles have a small specific surface area and exhibit a poor sintering property, involving a drawback that the sintering must be carried out at higher temperatures.
An object of the present invention is to provide ammonium aluminum carbonate hydroxide, a new compound which possesses commercially more desirable properties and permits production of .alpha.-alumina more easily than the conventional aluminum carbonate hydroxide compounds.
Another object of the present invention is to provide a method for the manufacture of ammonium aluminum carbonate hydroxide which is a new compound.
Still another object of the present invention is to provide a method for manufacturing a compact sintered alumina at notably low temperatures.